Circulators are well known in the prior art. A circulator is an active or passive three- or four-port device, in which a microwave or radio frequency signal entering any port is transmitted only to the next port in rotation.
Circulators have numerous uses. For example, one port of a three port circulator may be connected to an antenna, while a receiver is connected to a second port of the circulator to receive signals received by the antenna and with a transmitter connected to a third port of the circulator to supply transmit signals to the antenna. The transmit signals are isolated by the circulator from the receiver, which might otherwise be damaged by those transmit signals.
The prior art includes: “Active Circulators—The Realization of Circulators using Transistors” S. Tanaka, N. Shimomura, K. Ohtake, Proceeding of the IEEE, March. Vol. 53, Issue: 3, pages: 260˜267, 1965; “A 1.5-9.6 GHz Monolithic Active Quasi-Circulator in 0.18 um CMOS Technology” Shih-Chieh Shin, Jhih-Yu Huang, Kun You Lin and Huei Wang, IEEE Microwave and Wireless Components letters, Vol. 18, No 12 Dec. 2008; and “GaAs Monolithic Implementation of Active Circulator”, Mark A. Smith, IEEE Microwave Symposium 1988, which are incorporated herein by reference.
The prior art also includes U.S. patent application Ser. No. 14/554,995, filed Nov. 26, 2014, which describes the active circulator shown in FIG. 1, and which is incorporated herein as though set forth in full.
In the prior art configuration shown in FIG. 1, to satisfy the operating condition for signal circulation and isolation, only a limited gate voltage and certain values of resistor Rfb 18 and resistor Rcm 21 may be used. Typically, the output impedance of the FETs is assumed to be very high, and Vgs is limited for operation to a small transconductance range. The resistor Rfb 18 and resistor Rcm 21 values may be set within a 70 Ohm to 300 Ohm range. However, because the actual FET output impedances vary dynamically depending on the Vgs or Vds condition, such a dynamically changing output impedance can break the active circulator operating condition at certain bias points and it may also very easily cause an unwanted oscillation. The active circulator of FIG. 1 is further described below.
FIG. 2 shows an example of how an output impedance for a GaN HEMT with a gate length of 450 μm varies over Vgs for a fixed Vds=7V. As shown in FIG. 2, as the Vgs goes up from −3.1V to −2.7V, the output impedance at 1.0 GHz varies from 318 Ohms to 108 Ohms. Such a dynamic output impedance change may cause instability of the active circulator during the Vgs transition from the off-state, which is a Vgs less than −3.0V, to the bias point of approximately −2.7V, unless a complex stabilization circuit is added.
What is needed is an improved active circulator, with higher bandwidth and improved power handling capability, and which is stable, while operating in an energy efficient manner. The embodiments of the present disclosure answer these and other needs.